The family Poxviridae is subdivided into two subfamilies. Chordopoxvirinae infect vertebrates whereas the subfamily Entomopoxvirinae infect insects. At least six genera comprise the subfamily Chordopoxvirinae. Virions of all genera of chordopoxviruses are similar except those in the genus Parapoxvirus. The virions are complex i.e. they do not conform to the two types of symmetry found in most other viruses. There is a dumbbell shaped core wherein is contained the double stranded DNA genome of the virus. Associated with the core are a number of enzymes involved in transcription. There are two lateral bodies of unknown function that reside within the concavities of the dumbbell shaped core. These structures are enclosed by a membrane like structure. In addition, an envelope consisting of lipids and several viral specified polypeptides can be found on the small percentage of extracellular virus.
The virion infects cells by fusion with plasma membrane of the cell or via endocytosis, the process by which cells ingest foreign objects in a non-specific fashion Once internalized the outer membranes of the virus are released and the transcriptional machinery of the cores is activated. Thus begins the genetic expression of poxvirus functions.
Viral cores can be prepared chemically in vitro in the presence of nonionic detergents and reducing reagents (3,5,7). Differential centrifugation of the chemically disrupted virions allows the purification of cores away from disrupted membrane material.
Viral cores thus prepared are transcriptionally active and have been used as starting material for the purification of many enzymatic functions of the virion that are associated with transcription (7, 11-13). Significantly, however, viral cores are no longer infectious i.e. when contacted with susceptible cell substrates in vitro no infectious progeny virus is produced (3).
Vaccinia and other orthopoxviruses have been genetically engineered to express foreign genes of interest in vaccine production or for the production of biologically interesting molecules in vitro. Vaccinia has a broad host range for both in vitro cell substrates and in vivo susceptible hosts.
Fowlpox and other avipox viruses have been manipulated to express foreign genes. These recombinant viruses have been useful for the production of biologically active molecules in vitro, the production of species specific avian vaccines and, interestingly, for the production of vaccines useful in non-avian species. This last point is of interest because avipoxes are restricted to avian hosts for productive infections. Inability of avipoxes to productively replicate in non-avian species provides useful properties such as perceived safety in vaccination.
The basic technique of inserting foreign genes into live infectious poxvirus involves recombination between pox DNA sequences flanking a foreign genetic element in a donor plasmid and homologous sequences present in the rescuing poxvirus (8).
Specifically, the recombinant poxviruses are constructed in two steps known in the art and analogous to the methods for creating synthetic recombinants of the vaccinia virus described in U.S. Pat. No. 4,603,112, the disclosure of which patent is incorporated herein by reference.
First, the DNA gene sequence to be inserted into the virus, particularly an open reading frame from a non-pox source, is placed into an E. coli plasmid construct into which DNA homologous to a section of DNA of the poxvirus has been inserted. Separately, the DNA gene sequence to be inserted is ligated to a promoter. The promoter-gene linkage is positioned in the plasmid construct so that the promoter-gene linkage is flanked on both ends by DNA homologous to a region of pox DNA containing a nonessential locus. The resulting plasmid construct is then amplified by growth within E. coli bacteria (1) and isolated (2,6).
Second, the isolated plasmid containing the DNA gene sequence to be inserted is transfected into a cell culture, e.g. chick embryo fibroblasts, along with the poxvirus. Recombination between homologous pox DNA in the plasmid and the viral genome respectively gives a poxvirus modified by the presence, in a nonessential region of its genome, of foreign DNA sequences. The term "foreign" DNA designates exogenous DNA, particularly DNA from a non-pox source, that codes for gene products not ordinarily produced by the genome into which the exogenous DNA is placed.
Genetic recombination is in general the exchange of homologous sections of DNA between two strands of DNA. In certain viruses RNA may replace DNA. Homologous sections of nucleic acid are sections of nucleic acid (DNA or RNA) which have the same sequence of nucleotide bases.
Genetic recombination may take place naturally during the replication or manufacture of new viral genomes within the infected host cell. Thus, genetic recombination between viral genes may occur during the viral replication cycle that takes place in a host cell which is co-infected with two or more different viruses or other genetic constructs. A section of DNA from a first genome is used interchangeably in constructing the section of the genome of a second co-infecting virus in which the DNA is homologous with that of the first viral genome.
However, recombination can also take place between sections of DNA in different genomes that are not perfectly homologous. If one such section is from a first genome homologous with a section of another genome except for the presence within the first section of, for example, a genetic marker or a gene coding for an antigenic determinant inserted into a portion of the homologous DNA, recombination can still take place and the products of that recombination are then detectable by the presence of that genetic marker or gene in the recombinant viral genome.
Successful expression of the inserted DNA genetic sequence by the modified infectious virus requires two conditions. First, the insertion must be into a nonessential region of the virus in order that the modified virus remain viable. The second condition for expression of inserted DNA is the presence of a promoter in the proper relationship to the inserted DNA. The promoter must be placed so that it is located upstream from the DNA sequence to be expressed.
Thus, methods have been developed in the prior art that permit the creation of recombinant vaccinia viruses and avipox viruses by the insertion of DNA from any source (e.g. viral, prokaryotic, eukaryotic, synthetic) into a nonessential region of the vaccinia or avipox genome, including DNA sequences coding for the antigenic determinants of a pathogenic organism. Recombinant vaccinia viruses created by these methods have been used to induce specific immunity in mammals to a variety of mammalian pathogens, all as described in U.S. Pat. No. 4,603,112, incorporated herein by reference. Recombinant avipox viruses created by these methods have been used to induce specific immunity in avian species (10) and in non-avian species (9).
Unmodified vaccinia virus has a long history of relatively safe and effective use for inoculation against smallpox. However, before the eradication of smallpox, when unmodified vaccinia was widely administered, there was a modest but real risk of complications in the form of generalized vaccinia infection, especially by those suffering from eczema or immunosuppression. Another rare but possible complication that can result from vaccinia inoculation is post vaccination encephalitis. Most of these reactions resulted from inoculating individuals with skin diseases such as eczema or with impaired immune systems, or individuals in households with others who had eczema or impaired immunological responses. Vaccinia is a live virus, and is normally harmless to a healthy individual. However, it can be transmitted between individuals for several weeks after inoculation. If an individual with an impairment of the normal immune response is infected either by inoculation or by contagious transmission from a recently inoculated individual, the consequences can be serious.
Suitably modified poxvirus mutants carrying exogenous genes which are expressed in a host as an antigenic determinant eliciting the production by the host of antibodies to a host pathogen without productive replication of the poxvirus in the host represent novel vaccines which avoid the drawbacks of conventional vaccines employing killed or attenuated live organisms. Thus, for instance, the production of vaccines from killed organisms requires the growth of large quantities of the organisms followed by a treatment which will selectively destroy their infectivity without affecting their antigenicity. On the other hand, vaccines containing attenuated live organisms always present the possibility of a reversion of the attenuated organism to a pathogenic state. In contrast, when a recombinant poxvirus suitably modified is used as a vaccine, the possibility of reversion to a pathogenic organism is avoided since the poxvirus contains only the gene coding for the antigenic determinant of the disease-producing organism and not those genetic portions of the organism responsible for the replication of the pathogen.
Thus, it can be appreciated that a method which confers on the art the advantages of live virus inoculation but which reduces or eliminates the previously discussed problems would be a highly desirable advance over the current state of technology. This is even more important today with the advent of the disease known as acquired immune deficiency syndrome (AIDS). Victims of this disease suffer from severe immunological dysfunction and could easily be harmed by an otherwise safe live virus preparation if they came in contact with such virus either directly or via contact with a person recently immunized with a vaccine comprising such a live virus.